There are two basic structures for forming semiconductor devices, particularly for integrated circuits, using a group III-V semiconductor material, such as gallium arsenide. One structure comprises an outer layer of the semiconductor material of one conductivity type on a substrate body of a semi-insulating group III-V semiconductor material. The outer layer is generally epitaxially deposited on the substrate body. A buffer layer of the semiconductor material may be provided between the outer layer and the substrate body. Various semiconductor devices, such as field-effect transistors and millimeter wave transistors or diodes, are formed in the outer layer and electrically connected in a desired circuit. Another structure comprises regions of one conductivity type, such as n-type, in a substrate body of semi-insulating group III-V semiconductor material, such as gallium arsenide. The regions can be formed by ion implantation. The semiconductor devices are formed in the regions and electrically connected in a desired circuit.
In such integrated circuits it is necessary to provide isolation regions in the substrate body between the devices to electrically isolate the devices from each other. In the integrated circuit which comprises an active layer on a semi-insulating substrate body, electrical isolation can be achieved by etching grooves through the outer layer to the substrate to provide areas of the active layer which are separated from each other by the grooves. However, to etch the isolation grooves is a time consuming and expensive operation. Also, the surface of the device is no longer planar and thus it is difficult to form conductors on the device to connect the individual devices in a desired circuit. The device can be made planar by filling the grooves with an insulating material. However, this further increases the cost of making the device.
Isolation regions can also be achieved by implanting ions of such species as oxygen, proton, boron, argon, etc., into areas of the outer layer where isolation regions are desired. The mid-gap energy levels associated with the ion bombardment-induced damage are responsible for the compensation of free carriers and thus turn the implanted regions highly resistive. The technique of using damage-related compensation has several disadvantages. One of the disadvantages is that the high resistivity characteristic of the implanted region is not stable with respect to high temperature thermal processes. This happens because of the annealing out of the bombardment-induced damage in the implanted regions during high temperature treatments. Upon annealing, the sheet resistivity of these regions drops from values acceptable for isolation purposes to the original non-implanted value. The temperature at which the sheet resistivity returns to its original non-implanted value, and thus the implanted regions becomes ineffective for isolation, depends on the implanted species and is about 400.degree. C. for proton, 600.degree. C. for boron and 700.degree. C. for oxygen implants.
The return of high resistivity of the implanted regions formed by the implantation of the ions to the original non-implanted state upon high temperature processing imposes a limitation in using them for GaAs device processing. Processing of GaAs devices often requires high temperature steps (i.e., temperature steps of greater than 700.degree. C.). Therefore, these high temperature process steps have to be carried out prior to the isolation implant.
Another disadvantage of this ion implantation method for forming isolation regions is that the implanted ions become foreign chemical species to the host crystal. These species can potentially lead to deleterious generic impurity effect on devices subsequently built in the host crystal.
Therefore, it would be desirable to have an implant isolation technique with improved high temperature thermal stability of the implanted regions (i.e., stability at temperature up to 900.degree. C. which is typically the highest processing temperature for GaAs devices) to provide greater flexibility for process design. Also, it is desirable for the implanted species be of the type which do not perturb the electrical and crystal properties of the host material.